Disease, injury and/or surgery can result in localized tissue damage within a vessel and possibly occlusion within the vessel. Angioplasty is a procedure which may be used to lessen the amount of occlusion, whereby a balloon is inserted into an occluded vessel and subsequently inflated to dilate the occluded area. In some instances, the balloon may damage the vessel wall during inflation. As a result, in about 30 to 50% of the cases, the initial increase in the vessel dimensions may be followed by a localized re-narrowing (i.e., restenosis) of the vessel at the occluded area over a period of about three to six months. Restenosis may be the results of hyperplasia within the neointima, vascular remodeling within the vessel, or shrinkage in the overall vessel dimensions. To prevent re-narrowing of the vessel, expandable stents, for example, have been implanted at the site of occlusion (i.e., constriction site), so that the a pathway may be maintain for fluid to flow therethrough.
There are a variety of methods currently used to implant an expandable stent within a vessel. A commonly used method first mounts an expandable stent in a non-expanded state on a balloon portion of a catheter. Subsequently, the catheter is maneuvered, with the stent thereon, along the vessel to the constriction site. Once the stent is at the constriction site, the balloon is dilated to expand the stent, and thus enlarging the vessel at the site of constriction. However, as with angioplasty, restenosis may subsequently result at the constriction site, despite the presence of the stent thereat. In particular, tissue growth may occur across the pattern of perforation (such a pattern is typically necessary to permit expansion of the stent from a non-expanded state) on the stent and into the stent lumen.
To reduce or prevent the occurrence of restenosis, there are stent designs which incorporate a therapeutic drug into or onto the stent body, which drug may diffuse or be released after the placement of the stent into a vessel. In one design, the therapeutic drug is coated onto the surface of the stent body. As fluid flows across the surface of the stent, the coat degrades and releases the therapeutic drug from the stent. However, such a design may permit the drug to be released too quickly. Consequently, the desired reduction in the occurrence of restenosis may not result. In another design, the therapeutic drug is incorporated into a strip of biodegradable material, and the strip placed onto the body of, for instance, a non-degradable stent. In this design, as the strip biodegrades the therapeutic drug is released at a rate at which the strip biodegrades. However, the use of a biodegradable strip may not provide a sufficient amount of a therapeutic drug necessary to reduce or prevent the occurrence of restenosis. With other designs, the therapeutic drug may be incorporated throughout the body or within different biodegradable layers of a biodegradable stent, so that the drug can be released as the stent biodegrades. The incorporation of the therapeutic drug into the body of the biodegradable stent may increase the total amount of drug over that used in the strip embodiment. However, the drug concentration must be so balanced so as not to affect the polymeric make-up of the stent, and thus the ability of the stent to biodegrade over a sufficiently long period during which restenosis may occur. At such a concentration, the amount of therapeutic drug may not be sufficient to reduce or prevent restenosis over the period necessary.
Accordingly, it is desirable to provide an expandable stent which can store a sufficient amount of therapeutic drug, which can vary the concentration of the drug without compromising the characteristics of the stent, and which can release the drug over a sufficiently long amount of time in a sufficient concentration to reduce and/or prevent the occurrence of restenosis.